Efficient lingocellulose biomass conversion to ethanol results in a distillation whole stillage that has very fine residue particles (mostly lignin, protein and ash) that cannot be separated effectively or economically using typical separation equipment such as centrifuges due to the large volume of fluid in a typical plant. In known processes, the solid and liquid components are separated using a mechanical filter separator consisting of fine cloth presses squeezed together using hydraulic pressure created by electrical motors. The separated liquid portion from the filter press is then fed to an evaporator for further condensing and the solid portion to a conventional dryer or it is burnt as a wet energy material in a special boiler.
Whole stillage soluble solids left in the liquid portion are deleterious to recycling back into the lignocellulose biomass conversion process and must be removed and treated. Typical solid liquid separation technologies remove suspended solids only and allow the soluble solids to pass through the separation step. Treatment of lignocellulosic soluble solids (e.g. digestion) is not very effective in high yield enzymatic lingocellulose to ethanol conversion processes. Concentration of the soluble solids can be energy efficiently done with a multi effect evaporation train, but total water removal can not be achieved as the fluid becomes very vicious as the water is evaporated from the liquid. Thus, additional treatment such as an air dryer is required which consumes large amounts of energy and the additional treatment is complex due to the stickiness of the solids.
Equipment required to effectively perform solid liquid separation, such as a filter press, is expensive due to the large volume of biomass to process. Also, using a filter press is inefficient as it only removes about 50% of the liquid from the solids.
As such, the high moisture solids treated by filter press technology requires further drying in order for the solids to be used as a fuel in standard boiler combustion systems, otherwise the wet mixture can only be combusted using more complex fluidized bed combustion systems which are designed to handle higher amounts of water. The removal of liquid in this manner requires additional energy, usually in the form of heat energy derived from natural gas or coal, and electrical energy to run the hydraulic pumps that “squeeze” the solids in the filter press, which energy cannot be reused for other purposes effectively or economically, due to contamination of steam with air. When steam is contaminated with air, added energy is no longer fully reusable, since the steam then transfers heat to its surroundings at a lower temperature, making the transfer much less useful. As most of the work we to be done in an ethanol plant is to boil water, any condensing temperature below that of boiling water is not readily useful. This better understood from the below chart, which shows the impact of air in steam at various pressures.
Resulting Temperature - Air and Steam MixtureMixture0% AirPressure(Pure Steam)5% Air10% Air15% Air(psig)(bar)(° F.)(° C.)(° F.)(° C.)(° F.)(° C.)(° F.)(° C.)20.152191042161022131012109950.35227108225107222106219104100.7239115237114233112230110201.4259126256124252122249121
For example, when the mixture is dried for use in a standard combustion boiler and it is dried in a separate dryer, the steam used for drying cannot be recycled, since it contains too much air (normal condensing temperature is around 60 C as there is normally over 70% air), which means the drying costs are very high when using this standard drying technology.
Depending on the type of lignocellulosic feedstock, these drying and combustion systems may require thermal oxidizers to reduce odours emitted to the environment from volatile organic compounds within the mixture.
As such, the steps of evaporation and conventional drying require significant amounts of energy and post treatment and, consequently are expensive.
It is therefore desirable to provide a process for separation of solid and liquid components which requires less capital and is more energy efficient.